Best Answer tmeister , 05 November 2013 - 03:42 PM
You have 2 collapser sections right now in your install.
Flatten already comes with the section collapser-flat built-in in the theme, the error is trigger by the standalone section ( wp-content/plugins/collapser/sections/collapser ).
Please deactivate the standalone collapser section (http://your-install/...min/plugins.php ) and use the one in the theme.
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